In a lithography system, for example, photons or charged particles such as ions or electrons are used for illuminating and patterning the surface of a substrate such as a Silicon wafer. Due to the energy load of such photons or charged particles, the substrate is at least locally heated. In particular in a charged particle lithography system, such as a multi-beam charged particle lithography system, the impact of the charged particles may cause a significant heating of the substrate, in particular in conjunction with local impart of the charged particles on the substrate.
Various substrate holding devices have been proposed which suppress a temperature rise of the substrate, and thereby stabilizing the temperature of the exposed substrate.
Many of these holding devices rely on a thermal contact of the substrate with a coolant which is arranged to flow through the substrate holding device. One example of such a device is disclosed in U.S. Pat. No. 5,685,363, which describes a substrate holding device comprising a heat absorbing fluid chamber underneath a wafer or target to be exposed. This known substrate holding device comprises a heat absorbing fluid covered by a flexible sheet. In use, a substrate is pressed against the sheet by a substrate retainer, whereby the sheet, and thus the heat absorbing fluid, comes into intimate thermal contact with the rear face of the substrate to be stabilized in temperature.
Where, such as in a charged particle beam lithography system, the substrate is heated only locally, while the heat absorbing fluid extends underneath virtually along the entire rear face of the substrate, the layer of heat absorbing fluid in this known design, in addition to acting as a heat absorbent, acts as and forms a heat buffer.
In addition, the temperature stabilizing device as disclosed in U.S. Pat. No. 5,685,363 contains a heat absorbing fluid passage or coolant passage included in the stabilizing base, through which heat absorbing fluid flows in order to cool the substrate holding device, and to convey heat away from the substrate holding device. This allows to stabilize the temperature of the substrate holding device underneath the heat absorbing chamber and provides better control of the temperature at which the substrate holding device and the target are to be stabilized.
In lithographic exposure systems, often accommodated in a vacuum environment, such coolant passages are not preferred. One reason is that the associated coolant conduits hamper or disturb accurate positioning of the substrate, either directly or via hysteresis.
Within the field of lithography, patent publication US2005/0128449 teaches that a phase change material, so called PCM, can facilitate the removal of heat. A PCM can store a large amount of energy as latent heat during a phase change from a solid to a liquid. Advantageously, large quantities of thermal energy can be stored at a relatively constant temperature. Thus the use of a PCM can provide temperature stabilization by storing large amounts of thermal energy without significantly changing the temperature. PCM materials can be applied without coolant conduits while still securely controlling the temperature at which a target or substrate and substrate holding device are to be stabilized. Materials indicated for realizing such heat storing and stabilizing system comprise paraffin wax and Rubitherm™ PX. The PCM may be provided as a PCM powder or as a bound PCM.
This manner of combined heat storage and temperature control is based on a generally known principle utilizing phase change of a material at constant temperature. In applying this principle, as may further be known from a vast amount of literature, amongst which “A review of on phase change energy storage: materials and applications”, by Mohamed M. Farid et al (Energy Conversion and Management 45, 2004), suitable materials may normally be selected from a handbook. In order to provide storage of large amounts of thermal energy at the desired temperature, a person skilled in the art will search for materials which possess a relatively high heat of transition or latent heat at the desired temperature, in casu the temperature of stabilization. One such handbook is the “Handbook of chemistry & Physics”, which lists “thermodynamic properties of the elements” based on research published by the US Atomic Energy Commission, Report ANL-5750. Indicative of the popularity of certain materials amongst a large variety of PCMs is the selection of a paraffin (n-octadecane), gallium and tin for validating a numerical simulation of PCM behavior in “Numerical simulation of solid-liquid phase change phenomena” by Costa et al, 1991.
The patent publication US2008/0024743, in the name of the applicant, provides an example of a lithographic target exposure system showing such known temperature stabilization system, in which coolant conduits are omitted by the application of a PCM in the form of Hexadecane, for example. Hexadecane was selected for reason that its phase change temperature matches the upper end of a typical temperature range for coolant fluid used in semiconductor manufacturing, thereby preventing the temperature of the substrate heat buffer to deviate to an undesirable extent from other, normally liquid cooled parts of an industrial lithography system. In this respect the PCM temperature of Hexadecane may e.g. be taken to be about 291 K from “characterization of Alkanes and Paraffin Waxes for application as Phase Change Energy Storage Medium” by Himran and Suwono (Energy sources, vol. 16, 1994), while fab coolant liquid may be taken to be within a range from 286 to 291 K (55 to 65 F), from “Bringing energy efficiency to the fab” by Chen, Gautam and Weig (McKinsey on semiconductors, Autumn 2013).
While Hexadecane has the advantage of a phase change temperature matching an industrial operating temperature, at least an industrial coolant temperature, it appeared in practice to suffer from poor performance due to poor conductivity of heat, despite the use of measures to improve thermal conductivity between the target and the PCM as taught in this known, PCM based substrate temperature stabilization system.
Furthermore, U.S. Pat. No. 7,528,349 discloses a temperature stabilization system comprising a heat absorbing material disposed in thermal contact with a substrate. The heat absorbing material is characterized by a solid-liquid phase transition temperature that is in a desired temperature range for material processing the substrate. According to U.S. Pat. No. 7,528,349, the heat absorbing material may be provided as a flat layer dispose on top of a carrier, may be disposed to fill one or more depressions in the surface of the carrier or may be embedded in the carrier by filling recesses with the heat absorbing material. The heat absorbing material is arranged in direct contact with the substrate or with a suitable thermally conducting layer that is in sufficient thermal contact with both the substrate. Where, such as in a charged particle beam lithography system, the substrate is heated only locally, the resulting heat is absorbed locally by the heat absorbing material. Due to the absorption of heat, the heat absorbing material will at least partially undergo a phase transition substantially at the location where the charged particle beam impinges the substrate. This local phase transition results in a local expansion or contraction of the heat absorbing material. These local expansion or contraction produces undesired distortions or deformations of the substrate, which makes the temperature stabilizing system of U.S. Pat. No. 7,528,349 unsuitable for high resolution charged particle lithography.
The present invention hence seeks to provide a system, apparatus and/or method which provides means for an accurate temperature control of the system, apparatus and/or the substrate holding device by using a well heat conducting, generally a metallic phase change material, while still matching a temperature within the semiconductor standard range of coolant liquids. Standard metal materials have a phase change temperature remote from this desired operating range. Gallium, with a transition temperature of 303 K, is closest to the temperature range for coolant liquids used in the semiconductor manufacturing, but still deviates by 12 degrees. Other metallic-like materials may be selected from metallic based compound materials. Where such liquid metallic materials may exhibit a Gallium-like substance behavior, the present invention further seeks to optimize a PCM stabilized substrate support in its combined function as receptacle of such a liquid metallic compound and substrate temperature stabilizer, thereby providing a new design of such temperature stabilizing substrate support.
Equally, while the substrate holding device according to US 2008/0024743 provides a very compact and sophisticated manner for maintaining the substrate on top of the substrate holding device at a substantially constant temperature, it also proved to be difficult to manufacture such a substrate holding device and/or to obtain a carrier or heat conducting frame with highly accurate and reproducible dimensions suitable for use in a lithography system.
In addition or alternatively, it is an object of the invention to provide a design that is adapted to, at least deals with, the specific nature of metallic like phase change materials such as any of the various gallium compounds. It appears in practice that these materials tend to demonstrate an ice and water like behavior in their transition from solid to liquid, in that the volume in solid form often may be larger than in liquid form, causing poor thermal conductivity due to at least potential loss of direct contact between the upper layer of the holding device and the phase change material included underneath.
In addition or alternatively, it is an object of the present invention to provide an exposure method and apparatus therefor, which is provides an accurate temperature control of a substrate, in particular in an apparatus where an exposing unit for projecting electromagnetic radiation or particles onto said substrate, is arranged at such a close vicinity to said substrate that the exposing unit may thermally affect the substrate.
In addition or alternatively, it is an object of the present invention to provide a substrate holding device which at least partially obviates at least one of the above mentioned drawbacks of the substrate holding device of the prior art.